Scala: Occurrences with pairs, groupBy and mapValues

occurrences3.scala

/**
  * For each unique character in the list `chars`, it calculates the number of
  * times it occurs. times(List('a', 'b', 'a')) should return List(('a', 2), ('b', 1))
  */
def howManyTimes(w: String): List[(Char, Int)] = { // w = "Wooord"
  val grouped = w.groupBy(x => x.toLower) // Map(d -> d, w -> W, r -> r, o -> ooo)
  val occ = grouped.mapValues(_.length) // Map(d -> 1, w -> 1, r -> 1, o -> 3)
  occ.toList // List((d,1), (w,1), (r,1), (o,3))
}